In plate heat exchangers of the kind described above the heat transfer plates are often made of stainless steel, whereas the frame plates and the connection pipes for cost reasons are made of steel which is not stainless. For protection of a frame plate provided with holes and also connection pipes connected therewith from contact with a heat exchange fluid linings, which are preferably made of the same material as the heat transfer plates, are often arranged in the through holes of the frame plate and in the connection pipes. The linings have permanent connections with the outer heat transfer plate situated closest to the frame plate, around inlet and outlet ports thereof.
The above described lining arrangement in a plate heat exchanger leads to certain strength problems. The heat transfer plates and the linings are thin and intended to get into direct contact with the heat exchange fluids, which leads to the consequence that these parts of the plate heat exchanger will quickly adopt the temperature of the heat exchange fluids and, thereby, undergo quick changes in length. The frame plates, on the other hand, are considerably thicker than the heat transfer plates and the linings and do not get into direct contact with the heat exchange fluids. Thus, a frame plate with one of its sides is in contact with an outer heat transfer plate in said plate package and with its other side is in contact with ambient air. This makes the frame plates undergo a smaller and, above all, slower change in length than the heat transfer plates.
The different changes in length of the heat transfer plates and the frame plates make the linings, their connections with an outer heat transfer plate and the portions of the outer heat transfer plate, which surround the ports, to be subjected to large forces which can lead to material breakage with subsequent leakage in the heat exchanger. The forces will be extremely large if the heat transfer plates and the linings are made of austenitic stainless steel, which has a particularly large coefficient of linear expansion compared to steel which is not stainless and from which the frame plates are normally made.
One solution to the above described problem with material breakage in or at the linings in a plate heat exchanger of the above described kind is presented in WO 95/31687 A1. This known plate heat exchanger has connection pipes 10, which on their insides are provided with linings 11. The linings 11, which have permanent connections with the heat transfer plate 3 situated closest to a frame plate 6, are arranged in the connection pipes 10 with a gap between a respective lining 11 and a connection pipe. The connection pipes 10 and the linings 11 preferably have a length which is at least twice the diameter of the connection pipes. The length of the linings and said gaps give the linings a possibility of radial movement in their respective connection pipes. Thereby, the forces acting on the linings, on said permanent connections and on the outer heat transfer plate are reduced.
A general advantage of plate heat exchangers is their compact construction. However, the plate heat exchanger described in the above mentioned WO 95/31687 A1 has a drawback in this respect. Thus, according to WO 95/31687, the length of said connection pipes should be relatively large , e. g. twice the diameter of the connection pipes, for the linings to be able to move radially to a desired extent. In practice, even longer connection pipes are used; for instance a plate heat exchanger having heat transfer plates measuring 1750.times.750 mm and port holes with a diameter of 200 mm may be provided with connection pipes having a length of 800 mm. These rather long connection pipes make the plate heat exchanger less compact than normally desired. Moreover, it has proved that material failure sometimes come up also in plate heat exchangers designed in this way, in most cases round the ports of the outer heat transfer plate.